1. Field of the Invention
This invention relates to the use of the combination of chlorine dioxide and ozone for preoxidation or pathogen inactivation in purification of water supplies, such as municipal water supplies, which allows an overall decrease in the ozone dosage. A beneficial decrease in the bromate formation was observed after ozonation. An on-site energy consumption reduction was also observed due to reduced ozone generation.
The discovery that chlorine dioxide application before ozonation can reduce the bromate formation in the ozonation process was unexpected. This has significant benefit to municipal water supplies. It was also found that the addition of sodium chlorite in place of chlorine dioxide before ozonation achieved similar results on bromate formation reduction.
2. Description of Related Art
Use of chemicals in drinking water treatment is well established as an effective means for pathogen inactivation and disinfection. Hitherto, many chemical compounds have been reported for use in water reservoirs. These chemicals include various quaternary ammonium salts, copper salts, and oxidants, such as the classic chlorine sources oxidizing disinfectants, chlorine and hypochlorite, bromine sources, ozone or peroxy compounds, e.g. hydrogen peroxide and potassium peroxymonosulfate.
The use of these chemicals carries with it occasional disadvantages while being effective as bactericides and disinfectants. For example, chlorine at improper pH and concentration levels produces unfavorable tastes and odors, and irritation. The halogen family of elements has been closely linked to the chemical disinfection of water. Although many alternative chemical disinfectants have been evaluated (e.g. ozone and hydrogen peroxide), chlorine in the elemental or hypochlorite salt forms continue to perform a dominant role in the water treatment field.
Chlorine has a sustained popularity in this field for many years due to two main factors. The effective bactericidal action of free chlorine in water even at relatively low levels and the excellent equipment developed for use and handling of chlorine has added to its popularity of use. Most of the present day potable water treatment plants use some form of chlorine for disinfection and rely upon the maintenance of a residual of free available chlorine concentration in the finished water to insure ultimate delivery of a safe and sanitary product throughout the distribution system. The inherent reactivity of free available chlorine that provides its strong bactericidal action is exhibited in other ways, notably in its instability and gradual loss from aqueous solutions and its chemical reactivity with a wide variety of both inorganic and organic constituents found in raw water sources. As a result, the residual free available chlorine concentration in the finished water released from a modern treatment plant is generally maintained at a level of between about 1.0 and about 3.0 mg./liter, with the particular value chosen dependent upon the expected rate of disappearance and residence times involved in the distribution system. The levels required will vary with water quality, which is subject to weather and seasonal changes due to the effects of temperature, sunlight, etc. on reaction rates, solubilities, etc.
The results and performance of this invention shows the efficacy of ClO2 preoxidation at a treatment facility that uses ozone as an oxidant for many beneficial results, e.g. taste and odor control, or for achieving acceptable concentration*time (CT) values. The results will benefit numerous drinking water facilities that have either installed or plan to install ozonation facilities to provide maximum protection against waterborne pathogens such as Giardia and Cryptosporidium while maintaining lower levels of bromate in the treated water to meet the drinking water standards. The problem most frequently associated with ozonation is the increase in biodegradable organic matter that occurs when the ozone oxidizes naturally occurring organic matter, thus creating a need for a biologically active filter. Another problem with ozonation is that of bromate (BrO3−) formation from the oxidation of the bromide ion (Br−) in the source water. While pH adjustment and reducing the ozone (O3) dosages are effective in reducing bromate formation, the ozone demand of the water prior to ozonation must still be met before an ozone residual high enough to achieve concentration*time values can be attained. Bromate ion formation occurs faster than pathogen inactivation. Bromate ion formation is a function of the ozone dosage, and increases with increasing ozone dosages. Coastal areas, where raw water bromide levels tend to be high, exhibit this problem. There are other methods, such as reducing pH when ozonating or adding ammonia before ozonation, that maybe effective for bromate formation reduction. However, these methods have their own limitations.